Field of the Invention
The present invention concerns a method to generate raw data sets from double echo exposures (data acquisitions) with a magnetic resonance system, as the execution of such a method; and a magnetic resonance system for this.
Description of the Prior Art
New fields of application in magnetic resonance tomography are offered by the acquisition of magnetic resonance data (MR data) with very short echo times TE (for example TE<500 μs). It is thereby possible to depict substances or tissue that cannot be shown by means of conventional sequences—for example a (T)SE ((Turbo) Spin Echo) sequence or a GRE (Gradient Echo) sequence—since their T2 time (the relaxation of the transverse magnetization of this substance or tissue) is markedly shorter than the echo time, and thus a corresponding signal from these substances or tissues has already decayed at the point in time of acquisition. For example, with echo times that are in the range of the corresponding decay time, it is possible to depict bones, teeth or ice in an MR image, even though the T2 time of these objects is in a range of 30-80 μs.
One approach to enable short echo times is to scan (enter raw MR data into) k-space at respective data entry points by detection of the free induction decay (FID). Such a method is also designated as single point imaging since essentially only one raw data point in k-space is detected per RF excitation. One example of such a method for single point imaging is the RASP method (“Rapid Signal Point (RASP) Imaging”, O. Heid, M. Deimling, SMR, 3rd Annual Meeting, Page 684, 1995). According to the RASP method, a raw data point in k-space whose phase has been coded by gradients is read out at a fixed point in time after the RF excitation relative to the echo time TE. The gradients are modified by the magnetic resonance system for each raw data point or measurement point, and k-space is thus scanned point by point.
To create an image that depicts only substances or tissue which have a very short T2 time (such as bones), it is typical to implement the RASP method twice, for example, with the RASP method operating with such a short echo time TE in the first pass that the bones (for example) still supply a signal, and with the RASP method operating with a correspondingly longer echo time TE in the second pass, so that the bones no longer deliver a signal. Each pass of the RASP method respectively delivers an image, wherein the two images created in such a manner are subtracted from one another so that only tissues or substances that have a very short T2 time are still shown in the resulting difference image.